nwfilter: fix crash when adding non-existing nwfilter
    
    Adding non-existing nwfilter to a network interface device without any
    nwfilter specified will crash libvirt daemon with segfault. The reason is
    that the nwfilter is not found an libvirt will try to restore old
    nwfilter configuration but there is no nwfilter specified.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>